1. Field of the Invention
This invention relates to a method and apparatus for controlling the manufacture of inductive thin film magnetic recording heads and pertains particularly to an improved lapping control system for obtaining improved inductive heads.
2. Description of the Prior Art
In high speed data processing systems, magnetic disks have been employed for large memory capacity requirements. Data is read from and written onto disks through magnetic transducers commonly called "magnetic heads" which are positioned over the disks during the storage and retrieval of data on tracks of the disks. The requirements for higher data density on the magnetic disks have imposed a requirement to read and write more data on narrower tracks located on the disk. In order to achieve maximum efficiency from a head, its pole pieces must have a pole tip height dimension, commonly called "throat height", which must be maintained within a certain limited tolerance for generating a maximum amount of electrical signals for a given head.
The conventional method for producing a required throat height includes a lapping system in which an abrasive grinding surface accurately grinds the pole tips to a desired length. Suitable techniques for controlling the throat height during the lapping operation are described in commonly assigned U.S. Pat. No. 4,689,877. In this patent, a technique is employed for measuring the resistance of electrical lapping guide (ELG) structures formed on each side of a row of transducers being lapped to a final throat height. These structures are lapped along one dimension to the same extent as the transducer pole tips. The resistance of an ELG structure at any given time indicates the amount of material that has been removed and hence the resistance is an indication of the final throat height of the transducers being lapped.
Another improved system is described in U.S. Pat. No. 4,914,868 which discloses a lapping system which is particularly designed for lapping magnetoresistive (MR) transducers. The system measures the changing resistance of an MR element as the MR element is lapped as a means to determine when the desired MR element height is reached.
Various techniques for calibrating ELG structures to provide for tracking to the throat height have been published. However, the basic problem that the structures do not match the structure of the magnetic head, remains. For this reason, these techniques may not work to the precision required.
There have been suggestions that the magnetic head characteristics can be evaluated from a signal which is generated by the head itself. In the publication "Substrate Testing of Film Heads" by Jones et al, IEEE Trans. Magn., Vol. 17, No. 6, p. 2896-8 (1981), a description is given of a test by which a measured inductance versus d.c. current through the head can be used to characterize saturation of the heads pole tips. Published Unexamined Japanese patent Application No. 62-287408 utilizes a high frequency signal in addition to a d.c. bias current to periodically magnetically saturate a magnetic head and then senses the reduction of head impedance due to the saturation to determine the write characteristic of the head. The Kawakami et al publication, J. Appl. Phys., Vol. 61, No. 6, p. 4163-6 (1987) describes the use of a special magnetic head, in which the pole tips are closed, to determine the end point in a lapping operation by sensing the change in inductance when the pole tips are opened.
Improvements and refinements in these approaches are disclosed in commonly assigned U.S. Pat. No. 4,912,883. In this patent, it is disclosed that the write current which first saturates a magnetic head (ISAT) can be related to the throat height and head magnetic performance parameters, provided that the saturation takes place in the front region of the head.
These prior art techniques are useful so long as throat height is the controlling parameter. However, as the magnetic heads have become progressively smaller, the throat height, pole tip thickness, and gap thickness have all decreased. This has contributed to the fact that throat height is no longer the only important influence on ISAT characteristics. Now, wafer to wafer variations can cause significant departures from a pre-established ISAT correlation.
Many methods for fabricating inductive thin film magnetic heads have led to numerous electronic lapping guide designs, calibration techniques and bow compensation systems. While many of these proposals have resulted in tighter control of throat heights, none have ultimately provided a system for satisfying extremely tight throat height requirements when operating in a small slider, high wafer density, low cost environment. In this environment, throat height must be controlled to less than .+-.0.5 microns.
In order to meet these stringent requirements, the most precise electronic lapping guide (ELG) design calibration system and active bow compensation must be employed. In order to keep cost down, the wafer densities must be as high as possible which necessitates a small slider format with as many sliders per row and rows per wafer as possible. This eliminates dedicated ELG sites since they would compete with valuable areas designated for sliders.
To meet these objectives, ELGs must be placed in row parting kerfs for maximum population of head rows and the parting kerfs must be as narrow as parting methods will permit to further maximize head densities per row. Current requirements provide for slider widths of 1.7 mm with a height of 0.425 mm leaving a total parting kerf of 0.30 mm. After parting tolerances are considered, an area of only 0.20 mm.times.0.425 mm can be allocated for ELG designs. It should be noted that even smaller sliders with even smaller kerfs are in demand.
None of the prior art systems are capable of meeting these requirements. Moreover, none of the prior art systems can be adapted to this application without major alterations. For example, conventional ELG designs with conventional resistive calibration systems of the prior art will not fit in a 0.2 mm kerf.
Calibrated ISAT approaches also fail to meet the objectives outlined above. The ISAT systems do not have the effective response range to provide a means for active bow compensation feedback, where response kerfs must be active from around 10 microns from final throat height. The ISAT responses typically began at 2.5 microns from final throat height. The short response curve does not provide enough working distance to adequately straighten the bow before the final throat height target is reached, resulting in wider throat height range and lower yield. In addition, calibrated ISAT, like calibrated resistive lapping has an ELG size limitation. The best implementation of calibrated ISAT regarding throat height controls requires offset element and switching components that far exceed the 0.2 mm kerf area allotted for ELGs.
Therefore, there is a need for an improved inductive head lapping system.